With so many business ventures vying for a piece of the green economy pie, it’s hard for investors to know which firm will be the next SunPower and which will be the next Solyndra. AlwaysOn, a business media and social networking company in Silicon Valley, CA, has released its list of the 200 top private companies in greentech, handing top honors to the company C3, which makes software meant to track and manage energy use and emissions in commercial buildings.

Partnering with heavy-hitters such as Morgan Stanley, Hewlett-Packard and Goldman Sachs; the AlwaysOn editorial team has identified the top 200 private companies that are making big industry safer, more efficient and environmentally friendly, while also creating viable high-growth business models for the new green economy. This year’s “GoingGreen Global 200″ companies were selected for “developing game-changing approaches and technologies that are likely to push outside the bounds of existing markets and away from entrenched players.”

The team based their assessments on a set of five criteria: innovation, market potential, commercialization, stakeholder value, and media buzz. In industries ranging from renewable energy and energy efficiency to waste management and materials solutions, these companies are providing the technology platforms, energy sources, waste solutions and software services needed to make the transition to a clean energy future.

The winners of the competition will be honored at the fifth GoingGreen event, taking place today and tomorrow at San Francisco City Hall. The entire list of awardees can be found here–and below, we’ve listed eight that AlwaysOn considers ones to watch: